A scintillation crystal used in a logging tool is typically an elongated, regular structure having a substantial cylindrical volume. It detects passage of gamma rays which converts to light and are called scintillation events. Such a passing gamma ray strikes or activates the crystal at random location within the crystal to create a flash of light. This momentary flash occurs at a specific location within the body or volume of the crystal. Assume for purposes of discussion that it occurs at the midpoint of the crystal between two equidistant ends. Further assume that the crystal is formed with mirrored walls which prevent the light from escaping the crystal. In whatever fashion the emission occurs, it radiates outwardly and is reflected, and the scintillation event is observed at the PMT at the end of the crystal. Assume that there are two PMTs deployed at opposite ends of the scintillation crystal. The light which is transmitted through the crystal body (either directly or through reflection) has an intensity which relates to the energy of the emission event. There is some energy loss between the precise point at which the light emission originates and the PMT transducers located at the ends of the scintillation crystal. There is a loss in measured energy as a result of reflection; there is also a loss to the degree that the scintillation crystal is not perfectly transparent or is cloudy.
The location in the crystal body where the scintillation originates thus erroneously changes the measure of energy observed by the PMT transducer system. As an example, if a scintillation event occurs precisely in front of a PMT, there is substantially less loss because there is a direct short path from the location to the transducer. The error arising from loss of energy is thus somewhat a function of the length of the scintillation crystal and the length from the scintillation event to the transducer that observes the event. Through the use of this apparatus, energy loss, either in transmission or reflection in the scintillation crystal body, is compensated. Thus, energy resolution can be improved and is made substantially independent of the location within the crystal body where the scintillation event occurs. This improves the resolution ability of a logging system incorporating a scintillation crystal.
The present invention is, therefore, briefly summarized as an improved detection system incorporating a regularly shaped, enlongate crystal body. The scintillation crystal body is constructed between two ends, having an PMT installed at each end. The output pulses from each PMT are input to a circuit which forms an output indicative of the corrected emission event energy level, the correction substantially overcoming energy losses arising from transmission or reflection within the crystal body.